An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the optical cladding ng (i.e., nc>ng).
For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “alpha” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, an alpha, or a triangle. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
Generally speaking, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber. In a single-mode optical fiber, the signal propagates in a fundamental LP01 mode that is guided in the fiber core, while the higher order modes (e.g., the LP11 mode) are strongly attenuated. The typical diameter of a single-mode or multimode glass fiber is 125 microns. The core of a multimode optical fiber typically has a diameter of between about 50 microns and 62.5 microns, whereas the core of a single-mode optical fiber typically has a diameter of between about 6 microns and 9 microns. Multimode systems are generally less expensive than single-mode systems because multimode light sources, connectors, and maintenance can be obtained at a lower cost.
Multimode optical fibers are commonly used for short-distance applications requiring a broad bandwidth, such as local networks or LAN (local area network). Multimode optical fibers have been the subject of international standardization under the ITU-T G.651.1 recommendations, which, in particular, define criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The ITU-T G.651.1 standard is hereby incorporated by reference in its entirety.
In addition, the OM3 standard has been adopted to meet the demands of high-bandwidth applications (i.e., a data rate higher than 1 GbE) over long distances (i.e., distances greater than 300 meters). The OM3 standard is hereby incorporated by reference in its entirety. With the development of high-bandwidth applications, the average core diameter for multimode optical fibers has been reduced from 62.5 microns to 50 microns.
There has been increasing interest in using optical fibers in nuclear power plants and other radiation-rich environments, such as particle acceleration laboratories and satellites. For example, optical fibers may be used in optical data communication links, distributed sensors, plasma diagnostics, and instrumentation systems. In such applications, optical fibers typically transmit signals through noisy electromagnetic environments, high gamma ray dosages and/or dosage rates, and high neutron fluences.
Signals transmitted via optical fibers typically undergo optical losses (i.e., attenuation) that accumulate over the distance traveled. These transmission losses increase substantially when the optical fiber is subjected to ionizing radiation, such as beta, alpha, gamma, and/or X-rays.
Generally speaking, radiation affects the optical properties of an optical fiber in two ways.
The first is referred to as “radiation-induced attenuation” (RIA), which occurs when radiation creates defects in the silica of the optical fiber. These defects absorb the transmitted electromagnetic signals. Radiation-induced absorption, therefore, increases the attenuation experienced by an optical signal as it is transmitted along an optical fiber's length.
The second is referred to as a radiation-induced refractive index change, which occurs when radiation induces refractive index changes in portions of the optical fiber. These refractive index changes can compromise the bandwidth of the optical fiber, in turn compromising the reliability of an optical transmission system. Accordingly, optical fibers used in radiation-rich environments should exhibit good radiation resistance.
Therefore, a need exists for a multimode optical fiber having a high bandwidth and good radiation resistance. More particularly, a need exists for a high bandwidth multimode optical fiber that exhibits low radiation-induced attenuation.